Method for Forming a Structure

ABSTRACT

Method for constructing a line or dotted structure on a support, especially for constructing strip-like electrically conducting contacts on a semiconductor component such as a solar cell, by applying an electrically conducting paste-like substance containing a solvent adhering to a support and subsequent hardening of the substance. After the substance is applied to the support, a medium containing a polar molecule is applied on the support and/or the substance, through which the solvent contained in the substance is extracted.

The invention relates to a method for constructing a linear orpunctiform structure on a support, especially for constructingstrip-like electrically conducting contacts on a semi-conductor elementsuch as a solar cell by applying an electrically conducting paste-likesubstance containing a solvent adhering to the support and subsequenthardening of the substance.

In manufacturing electronic structural elements the application of fineelectrically conducting structures primarily takes place throughphysical or chemical gas phase precipitation, epitactic methods usingmasks or possibly with additional laser support. These techniques permitthe manufacture of very fine structures, even if these are hardly suitedfor an economical mass production for economic reasons.

Other methods for applying fine structures are, for example, screenprinting, roller printing or tampon printing. Even if these methodsappear attractive from economic perspectives, there is nonetheless thedisadvantage that only a restricted resolution of the structures isattainable, so that using it is suited only for larger electroniccomponents with coarser structures.

In manufacturing solar cells, there exists with reference to the sidefacing the irradiation the requirement of applying as fine electricallyconducting structures as possible, which assure good electricalconductivity as well as good electrical contact to the solar cell, thusto the support. This requirement is imposed against the background thatthe surface facing the irradiation is to be shaded as little aspossible. Nonetheless, in order to make possible good electricalconductivity with a high tapping, the corresponding leads must have alarge cross-section. In order to meet these demands, contacts arefrequently applied according to the state of the art using screenprinting. But a precondition for this is that the support has a levelsurface upon which linear or strip-like contacts are applied. Epitacticmethods are also applied.

With a further known method, depressions are introduced into a solarcell surface using a laser beam, which are subsequently filled with aconductive material using chemical precipitation methods (U.S. Pat. No.4,726,850). But an appropriate method is quite expensive and thereforecost-engendering.

An electrically conducting paste is applied to a support inWO-A-91/24934, whereby the paste is polymerized and stabilized with UVlight.

In order to construct a structure on a plasma screen, applying asubstance with a binding agent decomposing by heat, which is thenhardened by the action of temperature, is known according to U.S. Pat.No. 6,312,864.

U.S. Pat. No. 6,322,620 proposes the hardening of a substance on asupport through thermal treatment.

Printing methods for applying structures on a solar cell can be gatheredfrom the literature: “Screen-printed Rapid Thermal Processed (RTP)Selective Emitter Solar Cells Using a Single Diffusion Step,” 16^(th)European Photovoltaic Solar Energy Conference, May 1-5, 2000, Glasgow,UK, p. 1087-1090 and “High Quality Screen-printed and Fired-throughSilicon Nitride Rear Contacts for Bifacial Silicon Solar Cells,” 16^(th)European Photovoltaic Solar Energy Conference, May 1-5, 2000, Glasgow,UK, p. 1332-1335.

A paste is known from JP-A-63268773 which is solvent-free and contains anoble metal powder, fritted glass and metal oxide as well as bindingagents.

Printing methods have the basic disadvantage that no high layerthickness can be generated with a small line width. This leads to thedisadvantage that wider lines or a larger number of these are necessaryto attain the desired low resistance values. Consequently an undesirableshading takes place.

Finger writing systems, to the extent that these succeed in being usedin structuring solar cells, are basically suited for fulfilling therequirements. But the line width depends strongly upon the interactionbetween the substance to be applied, as a rule in the form of a paste,and the surface properties of the support. Paste and support basicallyinfluence the spreading out of the former after application. Moreover,the structures applied in the form of linear strips manifest thetendency to run. The structure applied becomes partly coarse by this.Fine openings of the finger writing systems can be selected in order toavoid these disadvantages, owing to which, nonetheless, the riskincreases that the strips to be applied tear. The high sensitivity ofthe process to the relevant process parameters therefore leads to thecorresponding technique being used but seldom.

The present invention is based upon the problem of developing further amethod of the known type mentioned, such that the electricallyconducting structures, such as printed conductors or contacts, can beapplied on a support, especially a solar cell, whereby the shading inrelation to the support should be minimized. At the same time, theresistance of the structure should be low. The method should also besuitable for mass production, especially in manufacturing electricallyconducting contacts on a semi-conductor element such as solar cells.

To solve the problem, the invention basically provides that a mediumcontaining a polar molecule is applied on the support and/or thesubstance after applying the substance, through which the solventcontained in the substance is extracted.

Moreover, the paste-like substance can be applied to the support, forexample, using screen printing, tampon printing, spraying techniques orpreferably using finger writing techniques.

In accordance with the invention, that fine printed conductors aremanufacturable and hardenable is attained by the application of liquidswith polar molecules (media) to the freshly applied paste structure, sothat the freshly applied paste material or paste substance are preventedor restricted from flowing together, or a drawing together of the pastesubstance takes place. The pastes consist of mixtures of binders,solvents, detergents and metal particles and glass-like and/or ceramicbodies.

The polar solvent particles are extracted out of the paste through theattractive interaction of the medium on the edge and on the surface ofthe paste. The interaction between the polar molecules of a medium witha dipole element, such as, for example, media containing water, alcohol,acetone and other polar molecules with the polar solvent out of thepaste is regarded as a generally valid approach. The solvent diffusesout of the paste in the edge region and leaves behind an edge zone poorin solvent. This corresponds to a hardening and in this way causes astabilizing of the printed conductor structure by the edge layer.

It was possible to establish that the width-height ratio of thestructures applied can be altered with the paste-like substance throughinteraction between the substance and the applied medium. With themedium, it is, in particular, a matter of a solution of polar moleculesthat can interact with the surface of the substance as well as with thatof the support.

Moreover, it can be brought about that the paste-like substance isaltered shortly after application to the substrate in its breadth-heightratio in cross-section in favor of greater height and lesser width byaddition of a solution containing polar molecules as the medium, so thatthe desired fineness of the punctiform or linear structure sought isset. Through the interaction of the substances involved, a finestructure of electrically conductive paste-like substances in accordancewith requirements can be applied, whereby methods which are to begathered from the state of the art are used for the application, hencescreen printing, tampon printing, spray techniques or in particularusing finger writing systems. Consequently, a desired fine structure canbe economically applied on a support such as a solar cell with highthroughput.

On the basis of the fine structure attainable through the theory of theinvention, hence narrow contact surfaces between paste-like substanceand support, a region results on the support along the substancestructure after sintering, which was contacted initially, hence beforesintering with the substance, but is subsequently exposed again by quasicontraction of the substance. This previously covered and then exposedagain region is optically recordable and characteristic for the theoryof the invention.

The amount of medium applied is decisive for the effectiveness anddurability of the effect. If too much medium is applied, it can occurthat the medium (for example, water, alcohol) creeps under the appliedpaste, that is, a hardening also takes place from the underside by therunning out of paste solvent. In this way, the adhesive effect of theprinted conductor to the substrate is destroyed. The goal is to generatea hard shell for preserving form stability and an adhering core of theprinted conductor for preserving the adhesion on the substrate. Sincethe separation of the solvent out of the paste also depends on theamount of medium on the contact surface, that is, on the surface of thepaste, this can be diminished by a foam. The medium is thereforeadministered in a diminished extent on the substrate, in that, forexample, a surfactant for frothing the medium is introduced.

The running of the paste-like substance is also attained in anothermanner by applying the medium containing the polar molecule to thesubstrate, whereby the forces of adhesion of the medium to the supportare greater than the forces of adhesion between the substance to thesupport. In this way a running or flowing out of the paste-likesubstance is inhibited, and this can even be pushed back. A quasicontraction of the applied substance manifests itself with theconsequence that a good height-width ratio arises.

Of course, care should be taken that the forces of adhesion between themedium and the support, on the one hand, and the substance and thesupport, on the other, are adjusted to each other such that aninfiltration of the substance does not occur, since otherwise adetachment of the substance from the support could take place.Consequently, it must be assured that a sufficiently good adhesion ofthe substance to the support exists. Nonetheless, at the same time, asufficiently rapid hardening of the surface of the substance should takeplace, whereby an adherent core region should remain.

In other words, it can be guaranteed by using the medium that a runningof the paste-like substance is prevented after application to thesupport, but at the same time a sufficient adhesion to the support isguaranteed, whereby if need be a contracting of the substance withoutsacrificing contact should be made possible.

With the support, it is in particular a question of a metal, preferablya semi-conductor material such as single and/or multiple crystallinesilicon, germanium, a thin layer of amorphous silicon or compoundsemi-conductors such as cadmium telluride or copper indium diselenide.These materials are used as basic material for the manufacture of solarcells. Moreover, it is a principal aspect of the invention to attainfine metallic structures on the side of solar cells facing theirradiation, with low shading and consequently greater efficiency in thetransformation of light into electrical energy. Of course, the theory ofthe invention should not be hereby restricted. Rather, the method of theinvention can also be used to generate fine electrically conductivestructures on insulators such as glass or ceramic substances. The use ofstructures of this type to heat areal supports can be mentioned by wayof example. Metallic materials provided with insulation layers on whichelectrical components can be connected up based upon the theory of theinvention.

The paste-like substance basically consists of one or more resins, oneor more solvents as well as metallic powders such as silver, aluminum orlead glass as a sintering aid as well as glass and/or glass ceramics asadhesion mediators to the support. The substance can as mentioned beapplied using screen printing, finger writing devices, tampon printing,etc., to the surface of the support. If a good wetting of the supportsurface takes place, thus with good adhesion between the substance andthe support, then a running off—also called bleeding—of the substanceoccurs according to the state of the art, so that there exists thetendency to widen the applied structure, such as lines or strips. Thistendency to widen can be partially counteracted by setting a highsubstance viscosity according to the state of the art. Moreover,thixotropic pastes can be used, the viscosity of which is reduced uponapplying a shearing force, but strongly influences in a force-freestate.

In contrast, it is provided in accordance with the main aspect of theinvention that the outflowing or bleeding is prevented or confined inthat a further medium such as a liquid or one containing such is appliedto the support surface. With the medium it is a matter of a liquid or afoam. An aerosol is likewise conceivable. In order, among other things,to increase the adhesion to the support, a surfactant is preferablyadmixed to the medium. If the adhesion of the medium to the support isgreater than that of the paste-like substance to the support, thesubstance is pushed back and a running or bleeding is prevented.

In particular, a surfactant medium such as water is used which cancontain a surfactant. The hardening process basically also functionswith water without surfactant. Water ensures hardening. The surfactantensures foam formation and therewith primarily the proper dosage of theamount of water. Therefore, finer structures can be applied with the aidof the foam.

As mentioned, a solvent with polar molecules such as water, alcohol,acetone is used as a medium, which likewise separates the polar solventout of the paste.

An attractive interaction with the solvent of the paste is generated bya polar solvent, which leads to the solvent of the paste beingextracted, that is, passes into the solvent of which the mediumconsists.

This may be explained in reference to the following example. The dipolemoment of water is 6.07×10³⁰ Cm. The dipole moment of the paste solvent,for example of alcohols, is 3.5-5.8×10³⁰ cm. When the dipole moments ofthe two solvents agree, an exchange occurs as a consequence of theconcentration gradient. To this extent, there is overwhelmingly anexchange of the two liquids, such that the alcohol in the water goesinto solution. Through the exchange of the polar molecules, a diminutionof the solvent proportion in the material occurs in the edge zone of thematerial, which leads to a contraction of the applied paste. This leadsto the form stability of the applied printed conductor.

Either an exchange or a repulsive interaction (as in the case ofbenzene, hexane or carbon disulfide, carbon tetrachloride (insoluble inwater, since dipole moment is 0) occurs at strongly differentconcentration gradients. Here the substance of the paste is even pressedtogether and a diminution of the printed conductor width occurs, ifsubsequently a better wetting of the substance by the medium exists. Thesurfactant in the medium does not take care of the froth formation. Itdiminishes the surface tension in relation to the substrate and bringsabout a better wetting of the substrate. The surfactant-containingmedium wets the substrate better than the paste, and therewith pressesthe paste together.

In the simplest case, the surfactant medium is a soap solution of waterand soap.

In order, nonetheless, to prevent an excessively strong rolling back ofthe paste-like substance, owing to which in the worst case adissociation of the printed conductor into individual drops andtherewith interruptions could arise, the paste-like substance can havecomponents that react chemically with the medium in order to form amechanically stable surface or skin. The skin stabilizes the shape ofthe paste-like substance and consequently diminishes drop formation.Nonetheless, unreacted material remains behind in the interior of thepaste-like substance, owing to which the adhesion of the paste-likesubstance on the surface of the substrate is maintained.

A reaction up to the core of the substance can even take place withthinly applied structures such as printed conductors with diameters of,for example, 100 μm, so that a complete hardening is attained. Thishardening leads to a stable shape inside a few seconds. Nonetheless, asmentioned, care must be taken that the adhesion between the substanceand the support is maintained as otherwise a detachment would takeplace.

Furthermore, water-soluble (polar) and water-insoluble (non-polar)solvents can be contained in the substance. The water-insoluble solventshere assure that the paste properties are maintained by the solvent,without this being extracted by the medium. In this way, a selectivehardening of the outer layer of the substance is possible. Aninfiltration of the substance by the medium can basically be preventedin this manner.

Independently of this, a drying or sintering takes place in the usualway following stabilization of the paste, that is, preventingoutflowing, to burn the metal particles of the substance into thesubstrate.

Forces of adhesion, cohesion and flow of the substance are crucial forthe height-width aspect ratio of the paste-like substance. Here forcesof adhesion between the substance and the surface of the support play abasic role. The organic components of the substance such as resins aswell as solvents are decisive for this.

Further details, advantages and features of the invention becomeapparent not only from the claims, the features to be inferred fromthese, but also on the basis of the embodiments to be inferred inconnection with the drawings as well as the associated explanations,wherein:

FIG. 1 depicts a cross-section through a linear structure, manufacturedfrom a paste-like substance,

FIG. 2 depicts a top view of a support with a linear structure adheringto the latter,

FIG. 3 depicts a diagrammatic representation of the linear structureapplied to the support following application to the support or followingsintering in accordance with FIG. 2,

FIG. 4 depicts a graph for clarification of the height-width ratio ofapplied structures,

FIG. 5 depicts diagrammatic representations of printed conductorsapplied to a support, and

FIG. 6 depicts diagrammatic representations for clarification of thecontraction of printed conductors on a support.

The theory of the invention is preferably used for improving theefficiency of solar cells on the basis of crystalline siliconsubstances, without, nonetheless, a restriction taking place hereby.

Silicon disks which are provided with a surface layer of SiO_(x) withx≦2, or Si₃N_(x), with x≦4, which serve to reduce the reflection to aradiation, are exposed to air or moist air. An activation of thesubstrate surface is to be determined in connection with a treatment ofthis type, which can be explained with the generation of electricallyactive centers on bonds of the silicon or the Si₃N_(x). Thecorresponding treatment takes place preferably at a temperature T withapproximately 30° C.≦T≦ca. 60° C. over a period of time t withpreferably ca. 15 minutes≦t≦ca. 90 minutes. The relative humidity shouldlie between 40% and 90%. Alternatively, an activation of the surface ofthe support, thus the solar cell, can be reinforced by exposure towardan ozone-containing atmosphere or by additional treatment with UV lightor by a corona discharge along the surface. An amplifying action canalso be attained by subsequent or parallel treatment with a coronadischarge.

A paste-like substance (hereinafter called paste for short) is appliedto the surface of the support to be correspondingly treated, thus in theembodiment, of the solar cell. According to one aspect of the theory ofthe invention, the electrically conducting paste has short-chainsubstances which have a more strongly electronegative end. In this way,the paste is placed in a state which makes it possible to bring thesurface charge of the paste, conditioned by the electronegative endafter application upon the support, in interaction with the activatedlayer on the support. According to the setting of the paste, that is,the strength of the interaction, the latter inclines to diverge on thesubstance, or to a smaller contact surface. The divergence of the pastetakes place on the basis of a small wetting angle between the paste andthe support surface. Without additional measures, this would lead to anundesired wide metallic structure, which, of course, would have goodadhesion properties in relation to the support surface. A running of thepaste would then be prevented when ejecting forces arose between thepaste and the support, owing to which a larger wetting angle wouldarise, which leads a capacity of the applied structures to detach due tothe lack of adhesion.

In accordance with the invention, the running is prevented or contained,on the one hand, by the addition of a further medium or, on the other, aloosening of the paste from the support is prevented, so that narrowstructures are attainable. The medium changes with the paste such that,on the one hand, a contraction of the applied paste structure takesplace, on the other, the solidity in relation to the support is notdisadvantageously influenced.

A liquid, preferably a foam, is used as a medium which is applied to thesupport immediately after application of the paste. Here the medium hasa composition such that forces of adhesion between this and the supportare greater than the forces of adhesion between the paste and thesupport. Conditioned in this way, the paste is quasi pushed back by themedium, with the consequence that a flowing apart is counteracted.Moreover, it could even be established that the structure is diminishedin breadth due to forces of adhesion at work, thus the paste is narrowedin its breadth.

Moreover, the paste contains components which bring it about that, afterapplying the paste on the support, at least one surface hardeningoccurs, with the consequence that a spreading is prevented, since thepaste applied, thus its shape, is retained by the surface hardening.

The paste can consist of several organic components such as resins andsolvents, various binders and deflocculation agents in solid and/orliquid form as well as organic solids, such as, for example, a mixtureof metals such as copper, silver, aluminum, lead, tin, soldering alloysas well as combinations from these and combinations of other, evennon-metallic substances such as aerosols of silicon oxide, siliconnitride, zinc oxide, soot/graphite particles or powders containing thesecomponents as well as inorganic sintering aids. Furthermore, lowtemperature melting glass alloys or inorganic substances such as glass,lead-containing glass, boron or phosphorus silicate glass, or metaloxides can be added as sintering aids, melting point lowering agents,adhesion mediators during the sintering process, flux, etc., to thepaste.

Furthermore, organic components such as polymers (resins) and organicsolvents are added. In particular, the organic components of a paste cancontain:

-   -   binders such as acrylic and polyester resins, chloropenes, ethyl        cellulose, ethylene, polyethylene, furanes, polyvinyl chloride,        alkyd resins, epoxide resins, epoxide esters, polyurethanes,        butadienes, silicones and mixtures thereof,    -   solvents such as dimethyl ester, butyl ester, water, alcohols,        ethanol, propanol, butanol, acetone, turpentine, benzene,        benzine,    -   cross-linking agents such as melamine, phenol resins,        benzoguanamine, urea cross-linking,    -   hardeners,    -   layer formers such as polyvinyl butyral,    -   wetting agents,    -   rheological components,    -   polymer modifiers such as plasticizers.

For example, the paste can consist of an ethyl cellulose binderdissolved in butyl diglycol, different binders of various chain lengthsand additives, which are supposed to possess an influence on the abilityof the paste to flow. A corresponding paste adjusted in suitableviscosity can be applied on the support such as the solar cell accordingto a suitable application method such as screen printing, model printingor finger writing methods.

If an application takes place according to finger writing technology,then the paste initially possesses a circular cross-section. The pasteis drawn out longitudinally by the motion of the support relative to theoutlet nozzle of the finger writing system. Moreover, the viscosity ofthe paste and the speed of the feed of the support should be set suchthat a paste thread with a diameter between 50 μm and 300 μm results,preferably 80 μm. Obviously, the finger writing system can also be movedin relation to the support, or these can be moved in relation to eachother.

In order to prevent the otherwise occurring unwanted widening of thestrip, measures proposed in accordance with the invention take placewithin the first seconds following application. Hence, the support orthe paste strip can be brought immediately after its application on thesupport into contact with:

-   -   a solid such as a fine power that binds the solvent to the paste        surface and forms a closable skin,    -   a liquid or a gas, either alone or in combination, which reacts        with the solvent or the resin of the paste,    -   electrical discharges which generate a plasma which leads to the        surface hardening of the paste,    -   electromagnetic radiation such as plastic-hardening UV light or        IR irradiation with rapid drying effect to harden the surface.

In particular, the paste strip is brought into contact with a liquidcontaining water, such as foam, immediately after its application on thesupport. The foam extracts the organic solvent out of the pastematerial, and in this way brings about a mechanical change such ashardening the surface. The surface consequently forms a tough skin orcrust, so that a further outflowing of the paste material is prevented.A firm crust is formed together with the solid content of the paste.

In the embodiment, the organic and at the same time water-solublesolvent butyl diglycol is removed up to 80% to 100% by contact with themedia such as surfactant-containing water solution. This leads to asurface hardening such as crust formation.

If, for example, pegs of a breadth of 100 μm are applied, then acontinuous hardening can occur. To prevent the completely hardened pegfrom being detached from the support, water insoluble solvents are alsoadded in accordance with the invention in addition to the water-solublesolvents, owing to which, on the one hand, the desired hardening takesplace and, on the other, the adhesion properties in relation to thesupport are retained.

In accordance with the invention, hardening and non-hardeningresin-solvent mixtures with admixture of metal powders are consequentlyprocessed into pastes in connection with the surfactant medium, whichharden in edge regions of a contact point in relation to the medium suchas water-containing liquid or foam. The components of the pastes aremixed together such that a portion of the paste contains a water-solublesolvent, the other portion a water-insoluble solvent. After applying thepaste mixture, the reaction with the medium such as foam leads toremoval of the water-soluble solvent from subregions of the paste andtherewith to hardening. The other portions of the paste remainunchanged, and, to be sure, those which contain water-insoluble solventsand consequently guarantee the adhesion properties toward the support.

According to a further proposal of the invention, a UV-hardeningcomponent can be added to the paste. Applying the paste to the supportis conducted under the action of UV light, owing to which a hardening ofthe paste takes place on its surface. A soft core remains in theinterior of the paste, which makes the original adhesion propertiespossible. The depth of penetration of the UV light lies in the range of100 nm to 1 μm according to the absorption properties of the organicadhesion mixture, whereby a further penetration of the radiation isprevented by the metal particles in the paste.

After applying the punctiform, linear or strip-like paste andinfluencing it in accordance with the invention to avoid a flowing outor running, a drying and subsequent sintering takes place in the usualmanner.

If one examined structures applied to a support in accordance with theinvention, then it can be established that the coverage of the pasteapplied originally upon a support after the sintering process covers asmaller surface of the support than directly after the application ofthe latter. This is made clear on the basis of FIG. 1 to 3.

Hence a diagrammatic representation of a punctiform structure 10 appliedto a support is reproduced in FIG. 1. The surface which the paste coversimmediately after application to the support is symbolized by the outercircle 12. The paste is pushed back from the support by the mediumapplied immediately after application of the paste to the support,especially in the form of a surfactant-containing foam, so that thestructure after sintering covers a surface 14 represented shaded inFIG. 1. A weak ring 16 enclosing the structure 14 is then recognizableon the support, which was originally covered by the paste material. Arefinement of the punctiform structure takes place through the method ofthe invention.

The same holds in relation to a linearly applied structure 18, as FIGS.2 and 3 make clear. FIG. 2 is a photograph of a paste-like strip appliedon a solar cell 20, the width of which after sintering corresponds tothe cross-hatched region 18 in accordance with FIG. 3. Originally thestrip has a breadth which is clarified in FIG. 3 by the dotted line 22,23 and in FIG. 2 by the strip 24, 26, in addition to the end structure18.

FIG. 5 basically is supposed to make clear how a printed conductor 38applied to a support 36 manifests the tendency to run, or how the latteris prevented or restricted on the basis of the theory of the invention.Hence the printed conductor 38 is represented in Representation Aimmediately after its application on support 36. Representation B, whichrepresents the printed conductor after a time Δt after application,makes clear the tendency of the printed conductor to widen.Correspondingly, manufactured printed conductor manifest, therefore, arelatively large width extension with the consequence that the support36 is considerably shaded. Now, it is provided in accordance with theinvention that after a time Δt after the application of the printedconductor, whereby the time span can lie in the region between 0.1 s and10 min., a medium containing a polar molecule is applied to the support36 as well as the printed conductor 38. With the medium containing thepolar molecules, it can be a matter of water, alcohol, acetone ormixtures of these, in order to mention media only by way of example.Since the paste substance of the printed conductor 38 contains solvent,this is at least partially extracted in to the medium, owing to which asolvent-free edge 42 arises in the paste, that is, in the printedconductor 38. The edge 42 is mechanically hard (stable). On the support36, a narrow zone 44 with paste material which continues to adhere thusexists in the bottom region of the printed conductor 38, so that thecontact to the support and the adhesion of the printed conductor 38 tothe support 36 is assured.

In Representation D, the printed conductor 38 is likewise provided witha medium containing a polar molecule after a time Δt followingapplication. That is, the medium 46 is applied to the support 36 and theprinted conductor 38. It is a matter of a foam with the medium 46 inExample D. In particular, the medium for forming the foam contains asurfactant. A dosage of the amount of medium to be applied takes placeconditioned by the foam, with the consequence that the amount of mediumon the edge 48 of the printed conductor 38, thus the surface of thepaste substance, is diminished, so that as a consequence of this, theinteraction in the edge zone of the paste substance with the medium isdiminished. Independent of this, however, an extraction of solvent fromthe substance this takes place, so that a narrow edge 48 becomesmechanically stable. At the same time, however, there results theadvantage that a larger adhering paste core 50 remains, which assures abetter adhesion on the support 36.

A further aspect of the invention is to be explained on the basis ofFIG. 6, which rests not only upon the thought that solvent is extractedfrom the substance for hardening, but uses the interaction of medium,which contains polar molecules, and the support, on the one hand, andthe substance and the support, on the other. Thus, a printed conductoris represented shortly after application to a support in FIG. 6A). Herethe printed conductor has a width b₀. After 1 minute, the width hascontracted by 12.5% in relation to the original width b₀, and after 3minutes by 17% in relation to the width b₀. This contraction was broughtabout by the fact that a medium is used whose forces of adhesion inrelation to the support are stronger than the adhesion of the substanceto the support. In order to achieve this, a substance (paste) is usedwhich contains, besides a polar solvent, a non-polar solvent which has adipole moment equal to zero or a very small dipole moment. For example,two pastes can be mixed, one paste of which contains a non-polar solventand the other of which contains a polar solvent. The diffusion out ofthe polar solvent does not lead to the complete hardening of thesubstance due to the presence of the non-polar solvent, that is, toincrusting of the edge of the printed conductor. The non-polar solventdiminishes the hardening effect on the edge and brings about adeformability of the printed conductor, as this is apparent from acomparison of FIGS. 6A), 6B) and 6C).

Furthermore, the proportions of the non-polar solvent in the substancealso act against the infiltration of the substance, thus of the printedconductor by the medium, since the adhesion properties of the unhardenedpaste core remain preserved.

The differences between strip-like structures applied on a supportaccording to the state of the art and the theory of the invention arelikewise to be clarified on the basis of FIG. 4.

Hence, height and breadth of strip-like contacts applied to a siliconsolar cell surface as a support are represented in FIG. 4, whereby thecurve 28 symbolizes a contact which has been applied according to thetheory of the invention and curve 30 according to such a state of theart. The units of height H and width B are selected differently inordinates and abscissas.

The contact 28 applied with a finger writing method on the silicon solarcell has a breadth of ca. 50 μm to 200 μm, preferably 90 μm to 110 μmwith a height H of 5 μm to 50 μm, preferably 30 μm. The side structures32, 34 which can be determined following sintering on the solar cellsurface, which correspond to the strips 24, 26 in FIG. 2, have adistance of ca. 150 μm with a height between 0.2 μm to 200 μm.

The aspect ratio between height and breadth reaches from 0.1 to ca. 1and typically is

-   -   33 μm:110 μm=0.3.        The contact 30 conventionally applied in the screen printing        method to a silicon solar cell in contrast has a breadth of ca.        50 μm to 300 μm, preferably 200 μm. The height of the peg runs        between 5 μm to 25 μm. Consequently, there results an aspect        ratio of height to breadth between 0.02 to 0.2 and typically        lies at    -   14 μm:200 μm=0.07.        On the average, a refinement of the line breadth by 20 μm to 50        μm in relation to the original application breadth can be        attained on the basis of the theory of the invention. The side        structures, which are to be attributed to the contraction of the        strip, are characteristic.

It should be stated by way of example, that, with an original breadth ofca. 130 μm, the final breadth, thus after sintering, comes to ca. 80 μm.In other words, by use of a surfactant-containing foam, this is soapplied that an elimination of the solvent from the paste and ahardening of the surface takes place in such a manner and in a period oftime that a reduction in width occurred in the previously indicatedevent.

The method of the invention can basically be used for various materialpairings in which a structure is to be applied using metal-containingpastes on metallic substances and a line width of ≦150 μm is to begenerated. This especially applies also for precipitated semiconductorlayers on amorphous silicon, cadmium telluride compounds or copperindium diselenide compounds on glass supports which have been previouslycovered with a conductive layer.

1. Method for constructing a linear and/or punctiform structure on asupport, especially for constructing strip-like electrically conductingcontacts on a semiconductor component such as a solar cell, by applyingan electrically conducting paste-like substance containing a solventadhering to a support and subsequent hardening of the substance,wherein, after the substance is applied to the support, a mediumcontaining a polar molecule is applied on the support and/or thesubstance, through which the solvent contained in the substance isextracted.
 2. Method according to claim 1, wherein the medium is appliedon the support to the extent that a flowing of the substance along thesupport is prevented or largely prevented while avoiding a detachment ofthe substance from the support.
 3. Method according to claim 1, whereinthe forces of the adhesion between the medium and the support aregreater than the forces of adhesion between the substance and thesupport.
 4. Method according to claim 1, wherein water and anionicsurfactants such as soap, fatty alcohol sulfates, alkyl benzenesulfonates and/or cationic surfactants such as invert soap and/oramphoteric surfactants and/or non-ionic surfactants such as non-carbonicacid ester of polyalcohols is used as the surfactant medium.
 5. Methodaccording to claim 4, wherein the surfactant medium is applied on thesupport in the form of a liquid or a foam in the region of the appliedpaste-like substance.
 6. Method according to claim 1, wherein thepaste-like substance is applied to the support preferably by means ofscreen printing, tampon printing, finger writing techniques and/orspraying techniques.
 7. Method according to claim 1, wherein the mediumis applied to the support and/or the substance within a time interval Δtafter applying the substance, whereby the time interval Δt comes to ca.0.1 seconds to ca. 600 seconds, preferably ca. 1 second to ca. 60seconds.
 8. Method according to claim 1, wherein the substance isapplied to the support such that a diameter d with in particular 15μm≦d≦300 μm, preferably ca. 80 μm, results.
 9. Method according to claim1, wherein water soluble and water insoluble solvents are added to thesubstance.
 10. Method according to claim 1, wherein the substance isapplied punctiform, linearly or strip-like to the support in such as waythat the hardened substance has a height to breadth ratio a with inparticular 0.1≦a≦1.0, especially a ca. 0.3.
 11. Method according toclaim 1, wherein a silicon substrate with a surface layer consisting ofsilicon oxide and/or silicon nitride is used as a support.
 12. Methodaccording to claim 1, wherein a concentration gradient between themedium to be applied to the support and/or the substance and thesubstance is set with respect to the solvents present in the substancesuch that the solvent of the substance is extracted from the medium.